The damaging effects of sunlight on skin are well documented. Contrary to what most people believe, it is not necessary that one sunbathe to suffer the ill-effects of excessive UV exposure. In fact, significant damage can be done just by routine day-to-day activities in the sunlight. Some scientists estimate that over 70 percent of the damage the sun inflicts on the average person's skin over a lifetime is the result of simply being outdoors or even sitting by a window.
The major short term hazard of prolonged exposure to sunlight is erythema (i.e. sunburn). The 290 to 320 nanometer wavelength ultraviolet radiation range, designated as the "UVB" wavelength range, tends to be the primary cause of erythema. The 320 to 400 nanometer wavelength ultraviolet radiation range, designated as the "UVA" wavelength range, also produces erythema.
In addition to the short term hazard or erythema, there are also long term hazards associated with UV radiation exposure. One of these long term hazards is malignant changes in the skin surface. Numerous epidemiologic studies demonstrate a strong relationship between sunlight exposure and human skin cancer.
Another long term hazard of ultraviolet radiation is premature aging of the skin. This condition is characterized by wrinkling and yellowing of the skin, along with other physical changes such as cracking, telangiectasis (spider vessels), solar keratoses (growths), ecchymoses (subcutaneous hemorrhagic lesions), and loss of elasticity (sagging). The adverse effects associated with exposure to UVA and UVB wavelength radiation are more fully discussed in DeSimone, "Sunscreen and Suntan Products", Handbook of Nonprescription Drugs, 7th Ed., Chapter 26, pp. 499-511 (American Pharmaceutical Association, Washington, D.C.; 1982); Grove and Forbes, "A Method for Evaluating the Photoprotection Action of Sunscreen Agents Against UV-A Radiation", International Journal of Cosmetic Science, 4, pp. 15-24 (1982); and U.S. Pat. No. 4,387,089, DePolo, issued Jun. 7, 1983; the disclosures of all of which are incorporated herein by reference. Hence, although the immediate effects of ultraviolet radiation may be cosmetically and socially gratifying, the long-term hazards are cumulative and potentially serious. It has been estimated that eighty percent of lifetime sun exposure occurs during multiple brief exposures not intended to produce tanning. Therefore, photoprotection during these exposures to ultraviolet radiation is necessary.
The fact that these effects are taken seriously by the general public is suggested by considering the sun protection products' market. This market has grown considerably in recent years and many new products are introduced each year. What used to be looked upon as a seasonal business is no longer. Sun protection compounds are now included in a diversity of personal care products, particularly cosmetic-type products which are worn on a daily basis.
Physical sunblock agents are commercially available to protect the skin from UV radiation. These agents scatter, reflect, and absorb ultraviolet radiation. See, Sayre, R. M. et. al., "Physical Sunscreens", J. Soc. Cosmet. Chem., vol. 41, no.2, pp. 103-109 (1990). Examples of physical sunblock agents include titanium dioxide and zinc oxide. However, compositions containing a high level of these agents are opaque, generally unattractive in color, and are viewed as unacceptable for usage on more than just the nose or tops of the ears. Furthermore, these agents are very susceptible to rub-off or wear-off resulting in little or no protection.
The most common agents for sun protection are sunscreens. These agents exert their effects through chemical means, i.e., they absorb ultraviolet radiation so that it cannot penetrate the skin. Sunscreens present the user with several problems. For example, they must be on the surface of the skin at the time of exposure to be effective. Sunscreens are preventative so one must anticipate being in the sun. To be most effective, sunscreens must be on the skin as a continuous uniform film. Delivering such a film to the uneven surface of the skin is very difficult.
Sunscreen-containing oil-in-water emulsions are the most popular form of photoprotection products in the U.S. market. This form is cosmetically pleasing, safe, cost effective and versatile. In general, however, oil-in-water emulsions containing oil-soluble sunscreens do not provide as good sunscreening efficiency when compared to water-in-oil emulsions containing oil soluble sunscreens. Unfortunately, water-in-oil emulsions, while providing good sunscreening efficiency, are at a disadvantage in terms of their cosmetic properties. It is therefore highly desirable to formulate oil-in-water photoprotection emulsions which possess superior cosmetic properties and improved sunscreening efficiency.
Without being limited by theory, it is believed that the specific oil-in-water dispersions of the present invention having a surface tension of greater than about 40 dynes/cm, when measured as a 10% (w/w) aqueous solution of the emulsion, which possess a wax component having a required HLB between about 1 and about 8 and an oil phase with a required HLB of between about 1 and about 8 will form continuous oil films when applied to skin and thereby provide increased SPF efficiency.
The compositions of the present invention provide improved film-forming as well as improved skin hydration for improved moisturizing and improved efficiency of the sunscreen active thereby requiring a significantly lower level of sunscreen active than heretofore believed necessary to provide a relatively high level of sunscreen efficacy. The compositions of the present invention further provide reduced sunscreen migration thereby reducing, for example, eye-stinging, by inhibiting the sunscreen active from invading the eye area.
It is therefore an object of the present invention to provide a topical composition in a stable form, the use of which will prevent both acute (erythema) and chronic (photoaging) effects of exposure to the sun.
It is further an object of the present invention to provide specific photoprotection emulsion compositions which provide improved sunscreen efficiency which have a surface tension of greater than about 40 dynes/cm, when measured on a 10% (w/w) aqueous solution of the emulsion, which possess a wax component having a required HLB between about 1 and about 8 and an oil phase with a required HLB of between about 1 and about 8.
It is a still further object of the present invention to provide a photoprotection composition which can be applied to the skin on a daily basis and provide significant photoprotection with relatively lower levels of sunscreen active.
These and other objects will become readily apparent from the detailed description that follows.